(1) Field of the Invention
The present invention relates to a method of fabricating printed circuit boards, and more particularly to a method of fabricating high density printed circuit boards having fine pads.
(2) Description of the Related Art
Generally, in order to prevent occurrence of solder bridging during the element mounting operation, a printed circuit board is provided with solder resists each formed between element mounting pads (hereinafter simply referred to as "pads"). As a method of forming a solder resist between the pads, a photolithography method is generally used. With this photolithography method, as shown in FIG. 1A, a predetermined circuit (not shown) and pads 2 are formed on a substrate 1. Then, as shown in FIG. 1B, a solder resist 3 of an optical hardening type is applied using such methods as screen printing, curtain coating, roll coating and spraying method. Next, as shown in FIG. 1C, a mask film 4 which allows transmission of light only at predetermined portions 5 is placed on the applied solder resist 3, which is followed by the exposure process. Thus, as shown in FIG. 1D, after the development and hardening, the solder resist 3 is formed between the pads 2.
For the exposure step in the conventional photolithography process, a light source which emits parallel light beams is generally used. However, as disclosed in Japanese Patent Application Kokai Publication No. Hei 4-97588, as a conventional solder resist formation method, a solder resist only in a via hole is exposed to scattering light during the exposure process and the solder resist is formed on an inner wall of the via hole in order to prevent the solder from sticking to an unintended portion during the element mounting operation.
However, in recent years, the miniaturization of mounting electronic elements has greatly advanced with the lead pitches being narrowed down as a consequence thereof. The demand brought about by this is that pads on printed circuit boards must be made smaller and finer. When the pads are made finer, the distance between the pads naturally is made narrower. For forming a solder resist between such fine pads by using a conventional technique, the only conceivable method is either (1) to narrow down the line width of the solder resist, or (2) to reduce the distance between the pad and the resist.
In order to ensure good adhesion between the solder resist and the substrate, the line width of the solder resist is required to be not less than 0.07 mm. If the adhesion between the solder resist and the substrate is in a poor state, the solder resist may peel off during the element mounting operation resulting in defective mounting.
When the expansion and contraction of the materials used, the expansion and contraction of the mask film, and the required precision in the alignment are taken into account, the distance between the pad and the solder resist is required to be not less than 0.06 mm.
One may attempt, by using the conventional technique, to form a solder resist between pads having a pitch of 0.3 mm (pad width being 0.15 mm and a distance between adjacent pads being 0.15 mm). In this case, if the priority is placed on the provision of a 0.06 mm distance between the solder resist and the pad, the width of the solder resist will become 0.03 mm so that the necessary adhesion cannot be attained. On the other hand, if the priority is placed on the attaining of a solder resist width of 0.07 mm, the distance between the pad and the solder resist becomes 0.04 mm, which leads to manufacturing fluctuations and, as shown in FIG. 2, results in a state wherein the solder resist 3 covers a portion of the pad 2.
Thus, with the conventional technique as explained above, it has not been possible to form a solder resist in between fine pads whose pitches are not larger than 0.3 mm.